This invention relates to the fabrication of semiconductor devices and, more particularly, to a method of doping bodies of semiconductor material by temperature gradient zone melting.
In the manufacture of semiconductor devices, it is usually necessary to alter or tailor the conductivity type of a body, or of parts of a body, of a semiconductor material. This alteration is provided by distributing atoms of conductivity modifying impurities in a selected region or selected regions of the body. Several techniques for achieving that distribution are widely practiced today. For example, doping is provided by alloying, diffusion, epitaxial growth, and the like. The choice among the various methods is made on the basis of such considerations as cost and the character of the junction desired. Alloying, for example, provides very sharp but shallow junctions. Diffusion can provide a deeper junction but it is less sharp.
Temperature gradient zone melting is a method of doping semiconductor material that can provide very abrupt junctions coupled with high dopant concentrations. Specifically, temperature gradient zone melting provides doped regions containing a dopant concentration up to the solid solubility limit of the dopant. Another virtue of temperature gradient zone melting is that doped regions of unusual configuration can be provided. Early descriptions of temperature gradient zone melting and some of its applications will be found in U.S. Pat. No. 2,813,048 issued to W. G. Pfann, and in his book, Zone Melting, copyright 1958 by John Wiley and Sons, Inc.
While temperature gradient zone melting was known as a method of doping semiconductors very early in the life of the semiconductor industry, certain considerations prevented early its adoption as a standard tool of the semiconductor device design engineer. One such concern has been the treatment of the surface through which the impurity is to enter the body of semiconductor material. Such characteristics as surface uniformity and means to restrain the lateral spread on the surface of the impurity prior to the initiation of migration have been of interest. For example, see U.S. Pat. Nos. 3,897,277 (Blumenfeld) and 3,899,362 (Cline, et al), both assigned to the present assignee.
It is therefore an object of this invention to provide a temperature gradient zone melting technique which simplifies surface treatment requirements.